Known for their asymmetric chelae, where one claw becomes grossly enlarged to become the snapping claw (see beginning picture, compare the claws in A); the smaller claw is referred to as the pincer. This feature is a cool example of environmental control over development. Much like in basal flatfish, which chela becomes the larger one isn’t genetically-determined. It’s simply the claw that is used the most by the juvenile alpheid that becomes the snapper. This is supported by two lines of evidence:
- The snapper only develops in later instars (Young et al., 1994), so after the claws get used and a preference is gained; and
- If the established snapper is experimentally removed, the remaining claw becomes enlarged as it naturally becomes preferred over the regenerating claw (Darby, 1934).
The snapper is a weapon (Versluis et al., 2000). It’s capable of being closed at very high speeds, producing a loud bang (via cavitation bubble, as with stomatopods) and ejecting a high-pressure water jet used to attack prey or to defend from predators. The snapper has special modifications to allow this phenomenon to happen. For example, the propus has a socket into which a plunger on the dactyl inserts, and this is what allows a water jet to be produced.
Alpheids are typically staunchly monogamous, forming long-lasting pairs that can be set-up even before the female is reproductively active (Nakashima, 1987). They construct and live in their burrows together, helping each other in foraging, and all the other crappiness involved in married life. The loving partners even behave differently towards each other than towards other shrimp (Schein, 1975). The evolution of this lifestyle is hypothesised as being related to the limited sexual receptivity of the females – they can only mate right after moulting, so forming a monogamous relationship before sexual maturity is the perfect way of mate-guarding by the males (Grafen & Ridley, 1983).
There are at least two sexual systems recorded in alpheids.
- Some are gonochoric, meaning adults are either males or females (Knowlton, 1980).
- Others are protandric hermaphrodites with primary males (Gherardi & Calloni, 1993). This means that they develop as males, but then some individuals will change sex (develop female gonads and reduce the male gonads); these are always smaller individuals, leading researchers to propose that this type of hermaphroditism works in conjunction with social systems and competition (Charnov & Anderson, 1989).
Alpheids are unique in that some of them exhibit parental care towards juvenile stages (Duffy & Macdonald, 1999), as opposed to the standard procedure of releasing planktonic larvae to disperse.
This is part of their general tendency to be social animals, dwelling in large groups inside cavities – between organismic rubble, in sand burrows, under rocks and boulders, inside sponges, anemones, and ascidians, and basically any other suitable shelter (Vannini, 1985). These “tribes” tend to stay put for a long time, and expansion can even take place when the dwelling is becoming too crowded.
These dwellings are often used mutualistically by other species of fish, crab, or stomatopod, who are welcomed by the alpheids as inquilines. The inquilines get the shelter, while the alpheids get leftover food. In some cases, typically with gobiid fishes, the association is obligatory.
Notable alpheids include several species in the Zuzalpheus and Synalpheus genera that are the only non-insect crustaceans known to be eusocial, like ants and termites (Duffy, 1996; Didderen et al., 2006). They form colonies of a few hundred individuals related by 2 generations of kinship, living in the canals of a host sponge and cooperating in its defence. Only a single female (the queen) breeds. This fulfills the criteria for eusociality – there is the sacrifice of reproductive ability, and extreme cooperation.
Arguably the most famous alpheids are the pistol shrimp, alpheids in the Alpheus genus. They’re famous for their surprisingly loud bang, as seen in the above video.
A comprehensive alpheid database, although it doesn’t seem to be getting updated anymore, can be found on the Alpheidae LifeDesk.
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